Post-deflection acceleration storage tube

ABSTRACT

A direct viewing bistable storage tube is described in which post-deflection acceleration of the writing electron beam is employed to provide a high writing rate during conventional nonstorage operation without affecting the storage operation by separating the flood electrons from the high acceleration field in space or time. This is achieved either by positioning the post-deflection acceleration electrode between the output of the beam deflection means and the flood gun so that the low velocity flood electrons do not pass through the post-deflection acceleration field, or by adjusting the voltage on the acceleration electrode to eliminate the post-deflection acceleration field during storage. In addition, the firstmentioned type of storage tube is provided with an improved write-through operation using a reduced current writing beam due to the post-deflection acceleration so that a nonstored image of high brightness is produced simultaneously with a stored image on the same area of a phosphor storage dielectric in such tube.

United States Patent 1 Massey et al.

[ 1 Feb. 20, 1973 [54] POST-DEFLECTION ACCELERATION STORAGE TUBE [75] Inventors: Ernst M. Massey; William C.

Brown, both of Portland, Oreg.

[73] Assignee: Tektronix, lnc., Beaverton, Oreg.

[22] Filed: July 20, 1970 [21] Appl. No.: 56,478

[52] US. Cl ..3l5/l2, 315/17, 313/68 R [51] Int. Cl. ..H0lj 29/41 [58] Field of Search..315/l4, 12, 17; 313/68 R, 68 A [56] References Cited UNITED STATES PATENTS 2/1969 Winningstad ..31 5/10 1/1967 Courtan 3/1968 7/1962 Primary ExaminerReuben Epstein Att0mey.Buckh0rn, Blore, Klarquist and Sparkman SWEEP RI THROUGH 57 ABSTRACT A direct viewing bistable storage tube is described in which post-deflection acceleration of the writing electron beam is employed to provide a high writing rate during conventional nonstorage operation without affecting the storage operation by separating the flood electrons from the high acceleration field in space or time. This is achieved either by positioning the postdeflection acceleration electrode between the output of the beam deflection means and the flood gun so that the low velocity flood electrons do not pass through the post-deflection acceleration field, or by adjusting the voltage on the acceleration electrode to eliminate the post-deflection acceleration field during storage. In addition, the first-mentioned type of storage tube is provided with an improved writethrough operation using a reduced current writing beam due to the post-deflection acceleration so that a nonstored image of high brightness is produced simultaneously with a stored image on the same area of a phosphor storage dielectric in such tube.

16 Claims, 3 Drawing Figures PATENTED BZ Tfl $717, 786

( y FIG. 1

0v .+|oov .WRITE STORE WRITE STORE v. ERNST M. MASSEY WILLIAM C, BROWN INVENTORS.

BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS ACCELERATION STORAGE TUBE BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to charge image storage apparatus, and in particular to direct viewing bistable storage tubes employing post-deflection acceleration of the wiring beam. Storage tubes made in accordance with the present invention are especially useful as the display device of a cathode ray oscilloscope.

Direct viewing bistable storage tubes includinga simplified storage target having a phosphor layer which serves as the storage dielectric and the fluorescent screen for storing a bistable charge image and emitting a corresponding light image when uniformly bombarded with low velocity flood electrons are known, as shown in U.S. Pat. No. 3,293,473 of R. H. Anderson, issued Dec. 20, 1966. It is also known to provide such storage tubes with a write-through operation in which a nonstored image and a stored image are displayed on the same target areas. Write-through may be achieved by duty cycle pulsing the writing beam on and off, or by dither movement of the writing beam around a closed path to prevent the charge image produced by such beam from reaching a sufficient voltage to cause bistable storage, as shown in U.S. Pat.-No. 3,430,093 of C. N. Winningstad, issued Feb. 25, 1969. The storage tube of the present invention is an improvement on the above-mentioned simplified storage tubes and differs therefrom primarily in the use of a post-deflectionacceleration electrode and isolating the low velocity flood electrons from the high voltage acceleration field of such electrode to enable conventional nonstorage operation with a higher writing rate without affecting the storage operation of such tube. The post-deflection acceleration of the writing beam also enables a more simplified write-through operation of high brightness in some embodiments of the present storage tube merely by reducing the current of such beam.

Conventional nonstorage operation of a cathode ray tube is improved by post-deflection acceleration of the electron beam since this enables the beam to be deflected by the deflection plates when the beam electrons are traveling at a slower velocity and also causes such beam to produce a light image of high brightness on the fluorescent screen by accelerating the beam after deflection through a high electrical field, on the order of about 5 to 30 kilovolts. Heretofore, it was thought that such post-deflection acceleration operation could not be employed in a bistable storage tube because the low velocity flood electrons in such tube must strike the storage target at about 200 volts acceleration or less, which would not be possible if they were transmitted through the post-deflection acceleration field. To provide bistable storage in the simplified tube of the present invention, the potential difference between the flood gun cathode and the target electrode must be less than the first crossover voltage on the secondary electron emission curve of the storage dielectric which, for P-l type phosphor, is on the order of +200 volts. Thus, if the low velocity flood electrons were transmitted through the high voltage gradient of the post-deflection acceleration field, they would immediately erase any charge image written on the storage target by uniformly charging the surface of the POST-DEFLECTION storage dielectric above the first crossover voltage, above which bistable storage is not possible.

Another problem which is overcome by the present invention is the distortion of the post-deflection acceleration field by the lower voltage of the flood guns which, of course, will adversely affect the geometry of the writing beam. This is avoided by positioning the flood guns outside of the region of the post-deflection acceleration field, or by cutting off the flood guns during writing. For these reasons, previous bistable storage tubes have not employed post-deflection acceleration.

It has been found that the above-mentioned disadvantages can be overcome either by positioning the post-deflection acceleration electrode between the flood gun and the output of the deflection plates, or by adjusting the voltage on the acceleration electrode to eliminate any post-deflection acceleration field during storage when the low velocity flood electrons pass by such acceleration electrode on their way to the storage target. Thus, the present invention separates or isolates the post-deflection acceleration field from the low velocity flood electrons either in space or in time.

It is, therefore, one object of the present invention to provide an improved direct viewing bistable storage tube employing post-deflection acceleration of the writing beam to increase the writing speed of the conventional nonstorage operation while also enabling bistable storage operation of such tube.

Another object of the invention is to provide such an improved storage tube in which the post-deflection acceleration field is isolated in space or time from the low velocity flood electrons.

A further object of the present invention is to provide such a storage tube in which the post-deflection acceleration electrode is positioned between the flood gun and the output of the writing beam deflection means.

An additional object of the present invention is to provide such a storage tube in which a voltage adjustment means is employed to reduce the voltage on the accelerating electrode and thereby eliminate the postdeflection acceleration field during storage.

Still another object of the invention is to provide such a storage tube which is capable of an improved write-through operation in which a nonstored image of high brightness is displayed simultaneously with a stored charge image.

A still further object of the present invention is to provide such a storage tube in which the post-deflection acceleration electrode is in the form of either a helical strip of resistance material or a ring of conducting material, such ring electrode being separated from the output of the deflection plates by a shield mesh electrode.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a schematic view of one embodiment of the storage tube of the present invention;

FIG. 2 is a schematic view of a second embodiment of the storage tube of the present invention; and

FIG. 3 is a schematic view of a third embodiment of the storage tube of the present invention and associated voltage adjustment means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, one embodiment of the charge image storage apparatus of the present invention includes a direct viewing bistable storage tube having a writing gun 12 including a cathode 14 at one end of the tube connected to a negative DC. voltage source of about 3 kilo-volts. The writing gun 12 produces a writing beam 16 of high velocity electrons which is transmitted through a pair of vertical deflection plates 18 and a pair of horizontal deflection plates 20 before striking a storage target 22 positioned at the opposite end of the tube.

The storage target 22 is supported on a light transparent glass faceplate 24 forming a portion of the evacuated envelope of the tube 10. A target electrode 26, formed by a light transparent electrically conductive layer 26 of tin oxide or other suitable material, is provided on the inner surface of the faceplate 24. A storage dielectric layer 28 of phosphor material, such as manganese activated zinc orthosilicate (Zn SiO,: Mn), commonly called P-l phosphor, is provided over the conductive layer 26. The phosphor storage dielectric may be in the form of an integral or undivided layer having a {sufficiently porous structure to enable secondary electrons emitted from one side of the phosphor layer to be transmitted through such layer to the other side where they are collected by the transparent conductive layer 26, as described in the above-mentioned U.S. Pat. No. 3,293,473.

One or more flood guns 30 are positioned within the envelope of the storage tube for uniformly bombarding the storage dielectric layer 28 with low velocity flood electrons. The flood gun may be a triode or diode type gun including a cathode 32. The flood electrons cause bistable storage of a charge image formed on the phosphor storage dielectric layer 28 by secondary electrori emission in a conventional manner.

A post-deflection acceleration electrode 34 is provided within the storage tube envelope between the flood guns 30 and the output of the writing beam deflection system, which may be external magnetic deflection coils or the internal electrostatic deflection plates 18 and 20 shown. The post-deflection acceleration electrode 34 may be in the form of a helical strip of high electrical resistance material, such as a graphite carbon compound, coated on the inner surface of the tube envelope. The front end 36 of the helix is connected to a low D.C. voltage of about zero volts which is the average potential of the deflection plates 18 and 20, while the rear end 38 of such helix is connected to a high positive DC. voltage of, for example, +l5 kilovolts. As a result, a post-deflection acceleration field of +15 kilovolts is provided by the electrode 34 for accelerating the writing beam 16 to a high velocity after deflection of such writing beam by deflection plates 18 and 20. The writing beam 16 is only accelerated through a low field of approximately +3 kilovolts due to the potential difference between the writing gun cathode 14 and the ground voltage on the final anode of the writing gun before deflection. This enables easier deflection of the electron beam while the high voltage post-deflection acceleration field of +15 kilovolts causes the writing beam 16 to strike the phosphor layer 18 with an extremely high velocity, thereby producing a corresponding light image of high brightness during conventional nonstorage operation of the tube even for extremely fast deflection speeds of such beam. Thus, the maximum writing speed of the tube during conventional nonstorage operation is greatly increased by post-deflection acceleration. The flood guns 30 may be reversed bias to cut off during such conventional nonstorage operation.

During the storage operation of the tube 10, the flood gun cathodes 32 and the target electrode 26 are provided with proper voltages so that the target electrode is about +175 volts positive with respect to such cathodes. In the above-described example, flood gun cathodes 32 may be provided with a DC. voltage of +15 kilovolts similar to the potential on the output end 38 of the acceleration electrode 34, while the target electrode 26 is provided with a voltage of +l5,l volts. As a result, when the writing beam 16 produces a charge image on the phosphor storage dielectric 28 whose potential is above the first crossover voltage of the secondary emission characteristic curve of the storage dielectric, the low velocity flood electrons will cause bistable storage of such charge image. This is achieved in a conventional manner by the flood electrons driving driving the potential of such charge image more positive to a first stable voltage near that of the collector or target electrode 26 and driving the potential of the unwritten storage dielectric areas down to a second stable voltage near that of the flood gun cathode. Thus, it can be seen that, for bistable storage operation, it is critical that the low velocity flood electrons only be accelerated through a small field of about +200 volts or less. This is made possible in spite of the extremely high voltage field of the post'deflection acceleration electrode 34 by positioning such acceleration electrode in front of the flood guns 30 along the path of the writing beam.

A pair of collimating electrodes 40 and 42 are provided as coatings of conductive material, such as aluminum or silver, on the inner surface of the tube envelope between the storage target and the flood guns. The collimating electrodes 40 and 42 are connected to intermediate voltages of +15,090 volts and +l5,080 volts. The collimating electrodes act to more uniformly distribute the low velocity flood electrons over the surface of the phosphor storage dielectric and to cause such flood electrons to strike the storage dielectric substantially at right angles thereto in order to improve uniformity and reduce trace shadowing.

The vertical deflection plates 18 are connected to the output of a vertical signal amplifier 44 whose input is connected to an input terminal 46 to which the input signal being investigated is applied. A portion of the input signal is transmitted to a trigger generator circuit 48 which produces an output trigger pulse that is transmitted to sweep generator circuit 50. This causes the sweep generator to produce a ramp voltage output at a time corresponding to the application of an input signal at input terminal 46. The ramp output voltage of the sweep generator 50 is applied to the horizontal deflection plates 20 at the same time the input signal is applied to the vertical deflection plates 16 is a conventional manner. Of course, the above-mentioned triggered sweep operation can be replaced by a conventional synchronous operation when a periodic input signal is applied to input terminal 46, in which case the sweep generator is operated in a free-running rather than a trigger mode.

During the storage operation of the tube previously described, it is also possible to display a nonstored write-through charge image on the same area of the phosphor storage dielectric where a stored charge image is being simultaneously displayed. This may be accomplished by reducing the current density of the writing beam 16 during formation of the writethrough" image by means of a switch 52 connected to the control grid 53 of the writing gun. In the store position of switch 52, a negative D.C. supply voltage of 3,050 volts is applied to the control grid 53, while in the write-through" position of such switch, a negative DC. voltage of about 3,070 volts is applied to such control grid. This increase in reverse bias voltage in the write-through position decreases the current of the writing beam 16 so that at the writing speed used it does not produce a charge image of sufficient potential to exceed the first crossover voltage. As a result, such write-through image is not stored by the bombardment of the target 22 with the low velocity flood electrons of the flood guns 30, even though such bombardment causes bistable storage of another charge image on such target.

This type of write-through operation was not satisfactory in previous storage tubes because of the low brightness of the write-through image due to the decreased current of the writing beam. However, the post-deflection acceleration field in the present tube provides such a high velocity to the writing beam that a write-through image of high brightness is still produced, in spite of the low current of such beam. As a result, both the nonstored write-through image and the stored image are produced with a high brightness. The write-through operation is extremely valuable when comparing the waveforms of two different signals. For example, one of such signals can be stored for the standard of comparison, while the waveform of the nonstored signal is changed by circuit adjustments to match that of the stored signal. This is a considerably simpler write-through technique than that disclosed in the above-discussed US. Pat. No. 3,430,093.

Another embodiment of the storage tube of the present invention is shown in FIG. 2. This embodiment is similar to that of FIG. 1 so that the same reference numerals have been employed to designate like parts and only the differences will be discussed. In the embodiment of FIG. 2, a different type of post-deflection acceleration electrode is employed. Thus, a ringshaped acceleration electrode 54 of conducting material, such as gold, aluminum or silver, connected to a DC. voltage of kilovolts is provided between the flood guns 30 and the output ends of the horizontal deflection plates 20. A mesh-shaped shield electrode 56 connected to a DC. voltage of zero volts is positioned over the output ends of the horizontal deflection plates to prevent the post-deflection acceleration field from distorting the field between such deflection plates. The shield electrode 56 may be in the form of a spherical mesh supported on the end of a cylindrical metal tube 58 surrounding the horizontal deflection plates.

As shown in FIG. 3, a third embodiment of the present invention includes a storage tube similar to that of FIG. 1 so that the same reference numerals have been employed to designate like parts. Unlike FIG. 1, the embodiment of FIG. 3 employs a post-deflection acceleration electrode 60 between the flood guns 30 and the storage target 22. The acceleration electrode 60 may be in the form of a helical strip of resistance material, like electrode 34 of FIG. 1. The voltages at the ends 62 and 64 of the post-deflection acceleration electrode 60 are adjusted to eliminate the post-deflection acceleration field during storage so that the low velocity flood electrons emitted by the flood guns 30 do not pass through such field. Thus, the front end 62 of the post-deflection acceleration electrode may be connected in common with the output anode of the flood gun 30 to a switch 66, while the rear end 64 of such helix is connected in common with the target electrode 26 to another switch 68 whose movable contact may be ganged to that of switch 66. The flood gun cathodes 32 are grounded in this embodiment. During writing, when the post-deflection acceleration field is present, the flood guns are cutoff by applying a DC bias voltage of zero volts to the output anode of such flood guns by switch 66. At the same time, the target electrode 26 and the rear end 64 of the helix are connected by switch 68 to a positive DC. voltage of +15 kilovolts in the write position of such switch. Thus, during writing, the writing beam 16 is accelerated after deflection through a post-deflection accelerating field of 15,000 volts and the flood guns are cut off to prevent the low velocity flood electrons from passing through such high field.

During storage, the switches 66 and 68 are moved to the store position whereby switch 66 connects the flood gun anodes and the front end 66 of the acceleration electrode 60 to a positive DC. voltage source of +100 volts, and switch 68 connects the target electrode 26 and the rear end 64 of such acceleration electrode to a positive DC. voltage of +l volts. This eliminates the high electrical field used for post-deflection acceleration and enables the low velocity flood electrons to bombard the storage dielectric 28 and cause bistable storage of a charge image formed thereon by the writing beam 16.

It should be noted that a very low electrical field of about +75 volts remains along the helix electrode 60 to enable such helix electrode to operate as a collimating electrode for the flood electrons, similar to electrodes 40 and 42 of FIG. 1. Of course, this low electrical field of less than volts cannot be considered a postdeflection acceleration field since such post-deflection field must be equal to or greater than the pre-deflection acceleration field applied to the writing beam prior to deflection which, in the case of the embodiments of FIGS. 1 and 2, is about +3,000 kilovolts.

From the above, it can be seen that the embodiment of FIGS. 1 and 2 both isolate the post-deflection acceleration field and the flood guns in space by positioning the acceleration electrode ahead of the flood guns. However, the embodiment of FIG. 3 isolates the flood guns from the post-deflection field in time by switching the flood guns off during writing when post acceleration occurs, and by switching the post-deflection acceleration field off during storage when the low velocity flood electrons are emitted. It should be understood that the switches 66 and 68 may be electronic switches, provided of course that the electronic switch used for switch 68 is a mercury vapor switch or some other device capable of withstanding the extremely high voltage differences in the two switch positions.

It will be obvious to those having ordinary skill in the art that many changes may be made in the abovedescribed preferred embodiments of the present invention without departing from the spirit of the invention. Therefore, the scope of the present invention should only be determined by the following claims.

We claim:

1. A direct viewing bistable storage tube apparatus in which the improvement comprises:

a storage target including a storage dielectric and a target electrode;

first electron gun means including a first cathode for producing a writing beam of high velocity electrons which bombard the storage target;

deflection means for deflecting the writing beam to form a charge image on the storage target; second electron gun means including a second cathode mounted between said deflection means and said target for bombarding the storage target substantially uniformly with low velocity electrons to enable bistable storage of the charge image; and

post deflection acceleration means including an acceleration electrode positioned between the deflection means and the storage target for accelerating the writing beam through a high electrical field after said beam is deflected, said acceleration field having a voltage gradient which is much greater than the voltage between said second cathode and said target electrode during storage,

and isolation means for isolating the acceleration field from the low velocity electrons so that. they do not pass through said field.

2. Storage tube apparatus in accordance with claim 1 in which the isolation means includes support means for supporting said accelerating electrode between the second gun means and the output of the deflection means so that said low velocity electrons are not transmitted through said acceleration field.

3. A storage tube apparatus in accordance with claim 1 in which the isolation means includes voltage adjustment means for reducing the voltage on said acceleration electrode to eliminate the post-deflection acceleration field substantially completely during the storage time when the flood electrons bombard the storage dielectric.

4. storage tube apparatus in accordance with claim 2 in which the storage dielectric is made of phosphor material.

5. A storage tube apparatus in accordance with claim 4 in which the phosphor material is provided as a layer on one side of a light transparent support member also having a target electrode layer on said one side.

6. A storage tube apparatus in accordance with claim 4 in which the second electron gun means includes at least one flood gun.

7. A storage tube apparatus in accordance with claim 4 in which the acceleration electrode is a helical strip of high electrical resistance coated on the inner surface of the envelope of such tube.

8. A storage tube apparatus in accordance with claim 4 in which the acceleration electrode is a ring of low resistance conducting material and which also includes a mesh electrode mounted adjacent the output of the deflection means to shield the deflection means from the high field between the mesh electrode and the ring electrode.

9. A bistable storage tube apparatus in accordance with claim 4 in which the storage dielectric is of phosphor material and which also includes writethrough means for reducing the current of the writing beam from that used to form a stored bistable charge image, and for forming a nonstored charge image on the phosphor storage dielectric with said writing beam of reduced current at the same time a stored bistable charge image is maintained on said storage dielectric by the low velocity electrons to cause the phosphor storage dielectric to simultaneously emit light images of both the stored and nonstored charge images.

10. A storage tube apparatus in accordance with claim 9 in which the write-through means includes means for increasing the reverse bias voltage applied between the cathode and control grid of the first electron gun to reduce the writing beam current.

11. A storage tube apparatus in accordance with claim 3 in which the acceleration electrode is a helical strip of high resistance material positioned between the second electron gun means and the storage target, and the voltage adjustment means includes a switch means for reducing the voltage on the end of said helical strip electrode closest to the storage target. I

12. A storage tube apparatus in accordance with claim 11 in which the other end of the helical strip electrode is provided with substantially the same potential as the output voltage of said second electron gun means.

13. A storage tube apparatus in accordance with claim 3 which includes control means for preventing the second gun means from bombarding the storage target with low velocity electrons during the formation of the charge image by the writing beam.

14. A storage tube apparatus in accordance with claim 13 in which the control means includes a voltage adjustment means for biasing the second gun means to cut off.

15. A storage tube apparatus in which the improvement comprises:

a storage target including a storage dielectric and a target electrode;

first electron gun means including a first cathode for producing a writing beam of high velocity electrons which bombard the storage target;

deflection means for deflecting the writing beam to form a charge image on the storage target;

second electron gun means including a second cathode mounted between said deflection means and said target for bombarding the storage target substantially uniformly with low velocity electrons of lower velocity than said high velocity electrons; post-deflection acceleration means including an acceleration electrode positioned between the output of the deflection means and the storage target for accelerating the writing beam through a high electrical field after said beam is deflected, said acceleration field having a total voltage gradient second electron gun means including a second cathode mounted between said deflection means and said target for bombarding the storage target substantially uniformly with low velocity electrons of lower velocity than said high velocity electrons;

post-deflection acceleration means including an acceleration electrode positioned between the output of the deflection means and the storage target for accelerating the writing beam through a high acceleration field after said beam is deflected, said acceleration field having a total voltage gradient which is much greater than the voltage difference between said second cathode and said target electrode during storage; and

isolation means for isolating the acceleration field in time from the low velocity electrons so that they do not pass through said field, said isolation means including voltage adjustment means for reducing the voltage on said acceleration electrode to subwhich is much greater than the voltage difference between said second cathode and said target electrode during storage; and

isolation mean for isolating the acceleration field in space from the low velocity electrons so that they do not pass through said field, said isolation means 10 including support means for supporting said accelerating electrode in front of said second cathode relative to said target to space said field from said low velocity electrons.

16. A storage tube apparatus in which the improvemerit comprises:

a storage target including a storage dielectric and a target electrode;

first electron gun means including a first cathode for producing a writing beam of high velocity elec stantially eliminate said acceleration field during trons which bombard the Storage target; the time when said flood electrons bombard said target.

deflection means for deflecting the writing beam to form a charge image on the storage target; 

1. A direct viewing bistable storage tube apparatus in which the improvement comprises: a storage target including a storage dielectric and a target electrode; first electron gun means including a first cathode for producing a writing beam of high velocity electrons which bombard the storage target; deflection means for deflecting the writing beam to form a charge image on the storage target; second electron gun means including a second cathode mounted between said deflection means and said target for bombarding the storage target substantially uniformly with low velocity electrons to enable bistable storage of the charge image; and post deflection acceleration means including an acceleration electrode positioned between the deflection means and the storage target for accelerating the writing beam through a high electrical field after said beam is deflected, said acceleration field having a voltage gradient which is much greater than the voltage between said second cathode and said target electrode during storage, and isolation means for isolating the acceleration field from the low velocity electrons so that they do not pass through said field.
 1. A direct viewing bistable storage tube apparatus in which the improvement comprises: a storage target including a storage dielectric and a target electrode; first electron gun means including a first cathode for producing a writing beam of high velocity electrons which bombard the storage target; deflection means for deflecting the writing beam to form a charge image on the storage target; second electron gun means including a second cathode mounted between said deflection means and said target for bombarding the storage target substantially uniformly with low velocity electrons to enable bistable storage of the charge image; and post deflection acceleration means including an acceleration electrode positioned between the deflection means and the storage target for accelerating the writing beam through a high electrical field after said beam is deflected, said acceleration field having a voltage gradient which is much greater than the voltage between said second cathode and said target electrode during storage, and isolation means for isolating the acceleration field from the low velocity electrons so that they do not pass through said field.
 2. Storage tube apparatus in accordance with claiM 1 in which the isolation means includes support means for supporting said accelerating electrode between the second gun means and the output of the deflection means so that said low velocity electrons are not transmitted through said acceleration field.
 3. A storage tube apparatus in accordance with claim 1 in which the isolation means includes voltage adjustment means for reducing the voltage on said acceleration electrode to eliminate the post-deflection acceleration field substantially completely during the storage time when the flood electrons bombard the storage dielectric.
 4. storage tube apparatus in accordance with claim 2 in which the storage dielectric is made of phosphor material.
 5. A storage tube apparatus in accordance with claim 4 in which the phosphor material is provided as a layer on one side of a light transparent support member also having a target electrode layer on said one side.
 6. A storage tube apparatus in accordance with claim 4 in which the second electron gun means includes at least one flood gun.
 7. A storage tube apparatus in accordance with claim 4 in which the acceleration electrode is a helical strip of high electrical resistance coated on the inner surface of the envelope of such tube.
 8. A storage tube apparatus in accordance with claim 4 in which the acceleration electrode is a ring of low resistance conducting material and which also includes a mesh electrode mounted adjacent the output of the deflection means to shield the deflection means from the high field between the mesh electrode and the ring electrode.
 9. A bistable storage tube apparatus in accordance with claim 4 in which the storage dielectric is of phosphor material and which also includes write-through means for reducing the current of the writing beam from that used to form a stored bistable charge image, and for forming a nonstored charge image on the phosphor storage dielectric with said writing beam of reduced current at the same time a stored bistable charge image is maintained on said storage dielectric by the low velocity electrons to cause the phosphor storage dielectric to simultaneously emit light images of both the stored and nonstored charge images.
 10. A storage tube apparatus in accordance with claim 9 in which the write-through means includes means for increasing the reverse bias voltage applied between the cathode and control grid of the first electron gun to reduce the writing beam current.
 11. A storage tube apparatus in accordance with claim 3 in which the acceleration electrode is a helical strip of high resistance material positioned between the second electron gun means and the storage target, and the voltage adjustment means includes a switch means for reducing the voltage on the end of said helical strip electrode closest to the storage target.
 12. A storage tube apparatus in accordance with claim 11 in which the other end of the helical strip electrode is provided with substantially the same potential as the output voltage of said second electron gun means.
 13. A storage tube apparatus in accordance with claim 3 which includes control means for preventing the second gun means from bombarding the storage target with low velocity electrons during the formation of the charge image by the writing beam.
 14. A storage tube apparatus in accordance with claim 13 in which the control means includes a voltage adjustment means for biasing the second gun means to cut off.
 15. A storage tube apparatus in which the improvement comprises: a storage target including a storage dielectric and a target electrode; first electron gun means including a first cathode for producing a writing beam of high velocity electrons which bombard the storage target; deflection means for deflecting the writing beam to form a charge image on the storage target; second electron gun means including a second cathode mounted between said deflection means and said target for bombarding the storage target substantially uniformly with low vElocity electrons of lower velocity than said high velocity electrons; post-deflection acceleration means including an acceleration electrode positioned between the output of the deflection means and the storage target for accelerating the writing beam through a high electrical field after said beam is deflected, said acceleration field having a total voltage gradient which is much greater than the voltage difference between said second cathode and said target electrode during storage; and isolation mean for isolating the acceleration field in space from the low velocity electrons so that they do not pass through said field, said isolation means including support means for supporting said accelerating electrode in front of said second cathode relative to said target to space said field from said low velocity electrons. 